A music therapist desires to instill various cognitive states in his patients, each according to their individual needs. He may wish to relax one patient, but agitate another; energize one, but frustrate another; etc. Additionally, it often is desired to instill a variety of changing cognitive states in a patient during a single session. All this is accomplished by choosing particular sequences of music to play to the patient which facilitate the desired mood.
It can be realized from common experience that different types of music evoke differing moods in the listener, ranging from agitation and displeasure, to pleasure, relaxation, and even boredom. Music theory has quantified this intuitive knowledge by identifying the parameters that define different types of music and their usual effect on the listener. A therapist attempts to apply this knowledge to clinical patients by varying one or more parameters to evoke a particular desired response.
An important quantifiable parameter of music is the degree to which it fulfills or violates the expectations of the patient. By means of the rules of melody, harmony, and rhythm in a general musical system, such as western tonal music, certain musical stimuli are more likely than others to follow a given musical sequence. Given a specific musical context, one can categorize the next musical stimulus in order of probability of occurrence, taking into account, of course, the specific styles of the music. What constitutes a major expectancy violation in one style of music might not be as significant an expectancy violation in another. One can therefore choose a particular type of musical stimulus from the list of possible stimuli whose probability of occurrence matches the value of the desired expectancy state. Expectancies in music are principally generated along harmonic, melodic and rhythmic dimensions. All these aspects can be expressed in separate probability distributions and incorporated into the creation of an overall auditory stimulus.
Music that generates an expectancy in the listener and then violates that expectancy creates tension, and is thus emotionally more challenging and potentially more distressing, than music which fulfills the expectancy. Thus, there are expectancy differences between dissonant music, which strays from the usual chord structures; melody sequences to which a listener in our society has become accustomed, and melodious and harmonious music, which retains simple and expected structures and sequences.
Given a series of musical stimuli, the music therapist can, from the principles of music theory and from the styles of music a person has been exposed to, predict the approximate expectancy value of a particular future stimulus, the expectancy value being the degree to which the listener perceives the stimulus as consonant with (or, conversely, deviant from) what he expects to hear. Thus the therapist can predict what emotional effect that musical stimulus will have on his patient. The therapist can therefore choose between different types of music to select the one that will evoke an emotional state in the patient most closely approximating the desired therapeutic emotional state. There exists, of course, a wide variety of musical choices for use in therapy, including classical, jazz, rock, hymns, and other generally available selections, as well as a growing body of music that is written specifically for therapeutic ends.
Though music therapy has proven to be of significant value, therapists are limited by the inexact science of predicting and assaying a listener's reaction to a particular musical sequence. A listener's actual reaction to a musical sequence almost invariably differs to some extent from the predicted reaction. In terms of expectancies, the degree of the actual expectancy fulfillment or violation differs from the predicted value. This is due to our incomplete understanding of the effect of numerous variables on expectancy violation/fulfillment, and in large part to the varying effects a single musical sequence will have on different individuals due to their distinct backgrounds and musical experiences. There exists, therefore, room for improvement in the efficacy of music therapy treatment.